The invention relates to a method of cutting a ribbon into signatures of a desired length and a corresponding cutting system.
In web-fed printing units it is necessary to cut a paper web, which is provided from a roll of paper, into single sheets. The paper web is also known as a ribbon, the sheets cut from the paper web are also called signatures. It is known to create signatures of different lengths with cutting cylinders having a fixed diameter by changing the velocity of the cutting cylinders relative to the velocity of the ribbon. A disadvantage of this technique results from the velocity difference between the cutting cylinder and the ribbon. With this technique, the circumferential velocity of the cutting cylinder must be equal to or greater than the velocity of the ribbon. If the circumferential velocity of the cutting cylinder is significantly greater than the velocity of the ribbon then the quality of the cut declines.
The transportation velocity of the signatures after the cut is related to the circumferential velocity of the cutting cylinders. As the circumferential velocity of the cutting cylinders increases relative to the velocity of the ribbon, the velocity of the signatures must also increase relative to the velocity of the ribbon. This situation requires that the signatures be accelerated to the new, higher velocity. The acceleration of the signatures can cause an inconsistency in the position of the signatures. This inconsistency in signature position can cause problems with the quality of the signatures and the performance of the cutting system and consequently with the performance of an entire printing unit.
From the article xe2x80x9cGoss exhibits futuristic concept pressxe2x80x9d by Gerry Valerio, it is also known to use a removable, seamless shell on a cutting cylinder in order to achieve a variable cutoff. The cutoff is changed by removing one shell from the cutting cylinder and installing another thicker or thinner shell. In order to compensate for the adjustments in the cylinder diameters, the center diameters of the cylinders have to be adjusted too.
It is accordingly an object of the invention to provide a method of cutting a ribbon as well as a cutting cylinder system which overcome the above-mentioned disadvantages of the heretofore-known methods and systems of this general type and which allow cutting signatures of different lengths without having to adjust the diameter of the cutting cylinder and which provide a good signature quality.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of cutting a ribbon, which comprises:
providing a cutting cylinder having a fixed diameter and rotating at an angular velocity;
providing a ribbon travelling at a constant velocity;
cutting the ribbon into signatures of a desired length with the cutting cylinder rotating at an angular cutting velocity determined by the constant velocity; and
changing the angular velocity of the cutting cylinder after the step of cutting in dependence of the desired length.
A ribbon is defined as any sheet-type or strip-type material, such as a paper web. A signature is defined as any part that is cut from the ribbon, such as a sheet of paper.
In accordance with another mode of the invention, the step of changing the angular velocity includes providing a mean angular velocity of the cutting cylinder which results in a circumferential velocity of the cutting cylinder that is either faster or slower than the constant velocity.
In accordance with a further mode of the invention, the angular cutting velocity is determined with the equation       W    cut    =            2      ·      π      ·      V        L  
with Wcut being the angular cutting velocity, L being the desired signature length, and V being the constant velocity of the ribbon.
In accordance with yet a further mode of the invention, a mating anvil cylinder rotating in synchronism with the cutting cylinder is provided.
In accordance with another mode of the invention, the step of changing the angular velocity of the cutting cylinder includes controlling the angular velocity of the cutting cylinder with the equation   W  =                              2          ·          π          ·          V          ·                      (                          L              -              N                        )                                    N          ·          L                    ·              cos        ⁡                  (                                                    2                ·                π                ·                V                            L                        ·            t                    )                      +                  2        ·        π        ·        V            L      
with W being the angular velocity of cutting cylinder, V being the constant velocity of the ribbon, L being the desired length of the signatures, N being a nominal signature length, and t being time.
With the objects of the invention in view there is also provided, a cutting cylinder system, comprising:
a cutting cylinder having a fixed diameter and rotating at an angular velocity for cutting a ribbon travelling at a constant velocity into signatures having a desired length;
a control device operatively connected to the cutting cylinder for controlling the angular velocity;
the cutting cylinder rotating at an angular cutting velocity during cutting operations, the angular cutting velocity determined by the constant velocity, and the cutting cylinder having a mean angular velocity different from the angular cutting velocity.
In accordance with another feature of the invention, the control device determines the angular cutting velocity with the equation       W    cut    =            2      ·      π      ·      V        L  
with Wcut being the angular cutting velocity, L being the desired signature length, and V being the constant velocity of the ribbon.
In accordance with yet another feature of the invention, the control device determines the angular velocity with the equation   W  =                              2          ·          π          ·          V          ·                      (                          L              -              N                        )                                    N          ·          L                    ·              cos        ⁡                  (                                                    2                ·                π                ·                V                            L                        ·            t                    )                      +                  2        ·        π        ·        V            L      
with W being the angular velocity of the cutting cylinder, V being the constant velocity of the ribbon, L being the desired length of the signatures, N being a nominal signature length, and t being time.
In accordance with a further feature of the invention, the cutting cylinder system further includes a mating anvil cylinder rotating in synchronism with the cutting cylinder.
In accordance with yet a further feature of the invention, the cutting cylinder system further comprises a variable speed motor for driving the cutting cylinder and the anvil cylinder, the control device controlling the motor.
In accordance with another feature of the invention, the cutting cylinder system further comprises a first variable speed motor driving the cutting cylinder and a second variable speed motor driving the anvil cylinder, the control device controlling the first and the second variable speed motor.
In accordance with yet another feature of the invention, the cutting cylinder system further comprises a mechanical linkage connecting the cutting cylinder and the anvil cylinder.
In accordance with the invention, the velocity of the cutting cylinder varies as it rotates such that the angular velocity of the cutting cylinder is a function of the angular position of the cutting cylinders, the desired signature length, and the velocity of the ribbon.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of cutting a ribbon and a cutting cylinder system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.